Light beam receivers are required where light beams are used for surveying. They are typically applied with, e.g., rotation lasers that are used on construction sites and similar. In order to be able to use the different types of radiated laser light, special light beam receivers are required. Examples of different types of radiated laser light include a punctiform beam rotating or in motion, a stationary or moving fan beam, or a laser plane fanned out by means of conic mirrors.
For example, if a punctiform horizontal laser beam rotates around an exactly vertically aligned axis of rotation, such light beam detectors can be used to carry out precise measurements of elevation. For this purpose photo-electric detector components are provided as detectors which, when laser beams are received, allow the receivers to measure the elevation independently of their position considering the radiated level of reference.
The photodetectors provided are usually embodied as one of the following: a quasi-linear detector line (as disclosed in U.S. Pat. No. 5,471,049); or a light conductor based PSD (photosensitive detector), as disclosed in U.S. Pat. No. 7,110,092 of the applicant; or an arrangement of several individual detector elements which are identical but, due to the respective height, of different electronic weighting regarding their sensitivity (as disclosed in U.S. Pat. No. 6,873,413); or, in the simplest case, two photo-electric devices of the same size arranged on top of each other.
More or less all of the photo-electric detector arrangements described above are suitable for height-resolving or location-resolving laser reception if supported by suitable evaluation means. However, when the practical application of these light beam receivers is concerned, it must be considered that they are usually exposed to sources of interference signals which can falsify the measuring results. In the worst case there might be a display of measurement results even though no laser beams were received.
On construction sites typical sources of interference are, e.g., fluorescent lamps, flash lamps at construction machines and light flashes emitted by electric welding apparatus. Although it has already been possible to sufficiently suppress the interference emitted by fluorescent lamps for many years (e.g., by high-pass filtering of the electric detector signals), the light beam receivers available on the market so far have offered only insufficient suppression of light flashes, and then the only alternative for the user had been to wait until the interference had disappeared.
Such interfering light flashes also contain energy of a wavelength range of usually 530 to 790 nm at which common construction lasers operate. Therefore, it is not possible to use only one simple optical filter (like the commonly used red or green optical filters) as part of the detector arrangement in order to effectively suppress these interfering light flashes.
Instead, a possible technique to suppress these interfering influences is described in US 2006/0082790. This document describes the use of an additional photo detector—to be mounted either below or above the detector line—which is located behind a separated window inside the housing of the light beam receiver. The sensitivity of the detector lies in the same wavelength range as the one of the detector line, thus being especially suitable for the laser pulses.
Here two cases can be assumed: either, the laser beam, i.e., the “wanted” signal, does not hit both the additional detector and the detector line itself at the same moment of real time; or, the intensities measured at the detector line and at the additional detector can be used to decide whether the laser reception nevertheless is interference-free. If not, an interference signal, or an interfered wanted signal, is present but it is not shown on the display.
Practical experience has shown that this procedure is very useful when it comes to suppressing light flashes caused by flash lamps. However, where strongly expanded laser beams are concerned, which regularly occur at larger distances and at poorly collimated lasers, it has been observed that these types of laser beams cannot be measured at the edge of the elevation measuring range, since the receiver mistakes them for an interference signal. This is due to the fact that strong portions of the signal hit the additional detector as well as the opposite side of the detector line. A possible solution would be to mount the additional detector at a larger distance either above or below the detector line. However, this possible solution is not preferable, as the dimensions of the device would be overly enlarged, and the necessity to mount the detector either above or below the detector line arrangement of the light beam receiver in the first place can already be regarded as a needless and impractical enlargement. A further disadvantage is the fact that such an additional detector would require an amount of electronic processing comparable to that of the detector line intended for the laser reception, including, e.g., variable gain amplifiers, peak detectors and integrators, or the like.